Why precision fit?

Abutment-screw loosening: just because it fits doesn't mean it works – Nobel Biocare

For every restoration type there are a variety of manufacturers providing all types of components. It can be difficult to know which components to choose and whether they will work together to ensure long-term restorative success. Before you commit to any kind of component, it is important to consider that restorations that may appear to fit but have not been carefully designed to work together can lead to complications. These complications range from screw loosening to implant breakage and system failure.

Screw tightening – a typical part of prosthetic aftercare

Prosthetic restorations should be checked regularly during control visits. However, as there are no commonly shared guidelines for prosthetic aftercare check-ups these can depend on the case and the clinician.

Sometimes technical complications occur or are discovered during the follow up. Porcelain chipping and screw loosening are among the most common types of complications. Repair should always be preceded by a thorough check of possible causes of the complication (e.g. overloading by incorrect occlusion) to prevent it happening again.

Unexpected abutment-screw loosening

There are cases where screw loosening occurs more often and the patient needs to come in for screw tightening outside the planned aftercare visits. These unexpected additional visits are inconvenient for the patient and increase the emotional burden linked with dental treatment. For the clinician, treating these patients means increased chair time and potential loss of business, as the time could have been spent taking care of a new paying customer.

While there is a lack of data related to the causes of screw loosening, in the second chapter of their recent e-book FOR presents factors that can contribute to screw loosening. Both finite element analysis and biomechanical testing indicate that several parameters can impact the performance of an implant system. System performance issues may not be visible to the naked eye; components may fit together but can still cause problems if not properly designed and tested to work together.

A Nobel Biocare abutment with a Nobel Biocare implant; forces are distributed evenly (left). A non-Nobel Biocare abutment with a Nobel Biocare implant may lead to uncontrolled force distribution (right).

The importance of prosthetic components on implant survival

Abutment-screw loosening has been cited as among the most common causes of implant-supported restoration failure, after osseointegration failure,1,2demonstrating that a system is only as strong as its weakest link.

How can you ensure a strong system? According to a recent article,3 accepted requirements for long-term maintenance and performance of implant cases include:

The use of properly matched components.

The precise fit of those components.

Sufficient preload on the clinical screws.

Adhering to these requisites helps ensure that implant components will not need to be retrieved or disassembled, screws will not loosen, and the implants themselves will not fracture once placed.

The science behind dental clinical screws

Insufficient preload leads to increased relative motion between the system components, which is a causative factor of screw loosening or even component failures. Conversely, preload values that are too high can result in fracture of the componentry.4In order to help avoid incorrect preload values, many manufacturers deliver a dedicated clinical screw with their abutments.

Nobel Biocare delivers clinical screws that have been optimized for the implant-abutment system that they’re part of. Depending on the abutment, connection type and platform size, screws are with or without coating, ensuring a strong and stable fit between the abutment and implant.

For example, diamond-like carbon (DLC), a coating for screws marketed under the brand TorqTite, shows higher preload values compared with screws that have a standard titanium surface (P<0.001).5

How to avoid screw loosening

Simple actions can make a big difference. Careful selection of the prosthetic components, the restoration and the screw will help. Additionally:

Always use the screw that is co-packed with the prosthetic component.

Never use lab screws in a clinical environment.

Use tooling intended to be used with the screws. Implant manufacturers have dedicated screwdrivers for different prosthetics; use the one that is intended for the chosen prosthetic.

Follow the manufacturer’s instructions on the intended tightening torque.

The implant-abutment system is interconnected, and the performance of any component depends not only on the component itself, but also on its interactions within the system. Therefore, choosing components that have been designed to fit and work together long term will be the safest way to ensure restorative success and patient satisfaction.